Analog to digital converter

ABSTRACT

An analog-to-digital converter (“ADC”) has an input terminal configured to receive an analog input voltage signal. A first ADC stage is coupled to the input terminal and is configured to output a first digital value corresponding to the analog input voltage signal and an analog residue signal corresponding to a difference between the first digital value and the analog input signal. A second ADC stage is coupled to the first ADC stage and is configured to convert the analog residue signal to a second digital value. At least one of the first ADC stage and the second ADC stage includes a first sub-stage configured to convert an analog signal to a first number of bits of a digital value representing the analog signal, and a second sub-stage configured to convert the analog signal to a second number of bits of the digital value, where the second number of bits is greater than the first number of bits. A controller is coupled to the first and second ADC stages and configured to combine the first digital value and the second digital value into a digital output signal representing the analog input voltage signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/720,613, filed on Sep. 29, 2017, the entire contents of which isincorporated by reference.

BACKGROUND

Analog-to-digital converters (“ADC”) are used in a variety ofapplications in order to convert a detected analog signal into a digitalsignal. Typically, the digital output is a binary number that isproportional to the input. The continuous analog input signal isperiodically sampled, and the output is a discrete digital signal.Converting a continuous analog signal to a digital signal requiresquantization of the analog input, which can introduce error.

Factors measuring ADC performance include conversion bandwidth anddynamic range (signal-noise-ratio), among other things. The bandwidth ofan ADC is characterized primarily by its sampling rate, and the dynamicrange of an ADC is influenced by factors such as resolution (number ofdiscrete values output over the range of the analog input values),linearity and accuracy (how well the quantization levels match the trueanalog signal), etc. The dynamic range of an ADC may be expressed interms of its effective number of bits.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram illustrating aspects of an analog-to-digitalconverter (“ADC”) system in accordance with some embodiments.

FIG. 2 is a block diagram illustrating further aspects of an example ADCsystem in accordance with some embodiments.

FIG. 3 is a circuit diagram illustrating further aspects of the ADCsystem illustrated in FIGS. 1 and 2.

FIG. 4 is a circuit diagram illustrating further aspects of the ADCsystem illustrated in FIGS. 1 and 2.

FIG. 5 is a timing diagram illustrating example timing signals forvarious ADC operation phases in accordance with some embodiments.

FIG. 6 is a timing diagram illustrating further example timing signalsfor ADC operation phases in accordance with some embodiments.

FIG. 7 is a circuit diagram illustrating aspects of an example coarsestage ADC in accordance with some embodiments.

FIG. 8 is a circuit diagram illustrating aspects of an example finestage ADC in accordance with some embodiments.

FIG. 9 a process flow diagram illustrating an example of an ADC methodin accordance with some embodiments

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Analog-to-digital converters (“ADC”) convert an analog signal into adigital signal. Typical ADC arrangements include pipelined, flash,Delta-Sigma, integrating (sloping), successive approximation register(“SAR”), etc. Pros and cons are associated with each ADC architecture.For instance, Delta-Sigma ADCs are capable of achieving relatively highresolution (16 bits and higher), but the achievable bandwidth may belimited.

SAR ADCs are typically able to operate with relatively low powerconsumption, but may have limited resolution. Increasing resolution andbandwidth with known SAR ADC arrangements can require undesirableincreased power consumption.

FIG. 1 generally illustrates an example of an ADC system 100 inaccordance with some disclosed embodiments. In general, the ADC system100 includes a first subranging ADC stage 10 and a second subranging ADCstage 20. An input terminal 102 is configured to receive an analog inputvoltage signal V_(IP), and the system 100 outputs a digital valueD_(out) representing the analog input signal V_(IP) at an outputterminal 104.

A first ADC stage 10 is coupled to the input terminal 104 and isconfigured to output a first digital value representing a portion of thedigital output D_(out), as well as an analog residue signal. A secondADC stage 20 is coupled to the first ADC stage 10 and converts theanalog residue signal to a second digital value, which represents theremaining portion of the digital output D_(out).

Referring to the example shown in FIG. 2, one or both of the first ADCstage 10 and/or the second ADC stage 20 include a first sub-stage, whichin the illustrated example is a coarse stage 110, configured to convertan analog voltage signal to a first number of digital output bits S1,and a second sub-stage, which in the illustrated example is a fine stage120, configured to convert the analog voltage signal to a second numberof bits S2, where the second number of bits may be greater than thefirst number of bits.

Thus, for the first ADC stage 10, the coarse stage 110 receives theanalog input voltage signal V_(IP), and converts that analog signal to adigital value having S1 bits (the most significant bits (MSBs) of thefirst ADC stage 10). The bits S1 generated by the coarse stage 110 arealso received by the fine stage 120, along with the analog input voltagesignal V_(IP). The fine stage 120 of the first ADC stage 10 then outputsS2 bits of the output (least significant bits (LSBs) of the first ADCstage 10 output). Additionally, the first ADC stage 10 outputs an analogresidue signal, which is the difference between the generated digitalvalue and the received analog signal.

Similarly, for the second ADC stage 20, the coarse stage 110 receivesthe analog residue signal, and converts that analog signal to a digitalvalue having S3 bits (the MSBs of the second ADC stage 20 output). Thebits S3 generated by the coarse stage 110 are also received by the finestage 120, along with the residue signal. The fine stage 120 of thesecond ADC stage 10 then generates S4 bits of the output (LSBs of thesecond ADC stage 20 output), which are combined with the MSBs from thefine stage 110 to generate the digital output of the second ADC stage20. The digital values generated by the first and second ADC stages 10,20 are then combined by a controller 30, which provides the digitaloutput signal D_(out) at the output terminal 104.

In some embodiments, the first and/or second ADC stages 10, 20 employsuccessive approximation register (SAR) ADCs, where a voltage range thatcontains the input voltage is successively narrowed. At each successivestep, a converter compares the input voltage to an output of an internaldigital to analog converter (DAC). At each step in this process, thisapproximation is stored in a register. However, the conversion rate ofknown SAR ADC arrangements is limited by its serial ADC operation.

The disclosed arrangement provides an analog to digital conversionfunction at lower power compared to conventional pipelined SAR ADCs, byreducing the accuracy requirements of the fine stage 120 by employingthe coarse stage 110, whose accuracy requirements can be significantlyreduced. The accuracy requirements of the coarse stage 110 is reducedfrom S2+S1 bits to S1 bits (S2>S1) by the use of a scheme discussedfurther below.

In accordance with aspects disclosed herein, the fine stage 120 is usedonly for a portion of the first and/or second ADC 10, 20 conversionperiod, thus reducing the overall power consumption, while maintainingthe benefits of high energy efficiency, high linearity and a fast SARbit cycling loop.

FIG. 3 illustrates an example ADC system 100 employing a pipelinedsubranging SAR ADC, where at least the first stage ADC 10 is a firstsubranging SAR sub-ADC. The first ADC stage 10 outputs N1 bits of thetotal digital output M bits, as well as an analog residue signalV_(RES). The residue signal, which represents the difference between theanalog input voltage signal V_(IP) and the digital value output by thefirst ADC stage 10, is output to a residue amplifier 40, which applies again factor to the residue signal V_(RES). The amplified residue isreceived by the second ADC stage 20.

An asynchronous SAR digital controller 130 receives an external clocksignal CLK_(in), whose frequency is related to the overall conversionrate of the ADC system 100. The controller 130 is configured toasynchronously control operations of the first ADC stage 10 as shown inFIG. 3. The coarse and fine stages 110, 120 include respective fine andcoarse comparators 112, 122. A reference voltage V_(REF) defines thefull scale analog signal range of the ADC system 100, which issuccessively narrowed by the coarse and fine stages 110, 120.

The coarse comparator 112 of the coarse stage 110 performs the MSBconversion of the first ADC stage. The resulting MSBs are received bythe controller 30, which initiates the LSBs conversion by the finecomparator 122 of the fine stage 120. The resulting LSBs are thencoupled to the controller 130, which combines the received MSBs and LSBsto output the N1 bits of the first stage to an alignment and digitalerror correction circuit implemented by the ADC controller 30.

The fine stage 120 of the first ADC stage 10 further generates thereside signal V_(RES), which is the difference between the analog inputsignal V_(IP) and the equivalent analog representation of the first ADCstage 10 digital output. The generated residue voltage V_(RES) iscoupled to the residue amplifier 40, where it is amplified by apre-determined gain “G.” The amplified residue is passed to the secondstage 20, where N2 bits of the total M bits of the digital outputD_(out) are generated.

As noted herein, in some examples the second ADC stage 20 includes thesame configuration as the first ADC stage 10. FIG. 4 illustrates anexample of further aspects of the second ADC stage 20. In the example ofFIG. 4, the second ADC stage 20 is a second subranging SAR sub-ADC. Thesecond ADC stage 20 outputs N2 bits of the total digital output M bitsbased on the received residue signal V_(RES), which represents thedifference between the analog input voltage signal V_(IP) and thedigital value output by the first ADC stage 10.

The asynchronous SAR digital controller 130 receives the clock signalCLK_(in), and is configured to asynchronously control operations of thesecond ADC stage 20 as shown in FIG. 4. The coarse and fine stages 110,120 of the second ADC stage 20 include respective fine and coarsecomparators 112, 122, which function to successively narrow the receivedreference voltage range V_(REF).

The coarse comparator 112 of the coarse stage 110 performs the MSBconversion of the second ADC stage 20, and the resulting MSBs arereceived by the controller 130, which initiates the LSBs conversion bythe fine comparator 122 of the fine stage 120. The resulting LSBs arethen coupled to the controller 130, which combines the received MSBs andLSBs to output the N2 bits of the second stage 20 to the alignment anddigital error correction circuit implemented by the ADC controller 30.

The accuracy requirements of the course stage 110 are much less than theaccuracy of the overall first and second ADC stages 10,20, therebyreducing the overall power consumption. Only the accuracy of the finestage 120 needs to be commensurate with the overall accuracy of the ADCsystem 100. Since the course stage 110 only converts the MSBs of thedigital value representing the received analog signal (V_(IP) orV_(RES)), the coarse stage ADC 110 only needs to be accurate to S1 bits.While the coarse stage 110 is converting the analog signal to the S1bits of the digital value, the second stage 120 can be idle, whichconserves power.

In addition, the conversion rate of disclosed ADC system 100 is improvedby the pipelined operation, which serves to mitigate the effects of theserial operation of traditional SAR ADCs.

FIG. 5 illustrates an example timing diagram for the first ADC stage 10and/or the second ADC stage 20. The timing diagram shown in FIG. 5illustrates time signals for a first operation phase phi1, a secondoperation phase phi2 and a third operation phase phi3 of the ADC system100. When the phi1 pulse is high, the first ADC stage 10 tracks andacquires the analog input signal V_(IP). At the same time, the residueamplifier 40 is disabled. Also, when the phi1 pulse is high, the secondADC stage 20 performs analog to digital conversion of the previouslysampled signal.

When the phi2 pulse is high, the first ADC stage 10 is performing analogto digital conversion of the analog signals previously acquired duringthe first phase phi1. At the same time, the residue amplifier 40 can bein an auto-zero mode, which is a circuit technique used to cancel theinput offset of an amplifer such as the residue amplifier 40. Also, whenthe phi2 signal is high, the second ADC stage is also performing analogto digital conversion.

When the phi3 pulse is high, the first ADC stage 10 holds the residuesignal, which is the difference between the input signal V_(IP) sampledduring phi1 and the equivalent digital output of the first ADC stage 10.At the same time, the residue amplifier 40 amplifies the analog residuesignal V_(RES) by a pre-determined gain value G. Also, the second ADCstage 20 samples the amplified residue corresponding to apreviously-generated residue signal.

FIG. 6 is another timing diagram showing the phi1 signal (track andhold), and ϕ_(c-stage) and ϕ_(F-stage) pulses, which are derived fromthe phi2 signal. The ϕ_(c-stage) and ϕ_(F-stage) pulses indicate thecoarse stage 110 and fine stage 120 successive approximation cycling ofthe SAR ADCs, respectively. As shown in FIG. 6, the fine stage SARcycling (generating the S2/S4 bits) begins following the course stageSAR cycling (generating the S1/S3 bits).

FIG. 7 illustrates further details of an example of the course stage110. Referring to FIG. 7 and the timing diagram of FIG. 6, the coursestage 110 includes a plurality of capacitors 150. Switches 152selectively connect one side of the capacitors 150 to the analog inputvoltage signal V_(IP) or the V_(REF) signal (based on switches 154), orground. The other side of the capacitors 150 are selectively connectedto a common mode voltage signal V_(CM) based on switches 156. Theswitches 154 and 156 operate in response to the phi1 signal, and theswitches 152 operate in response to the ϕ_(c-stage) pulses.

When the phi1 signal is high, the analog input signal V_(IP) is acquiredon the top plates of the capacitors 150. At the same time, the bottomplates of the capacitors 150 and the inputs of the course comparator 114are coupled to the common mode voltage V_(CM).

During the next phase, the ϕ_(c-stage) pulses are asserted to controlthe SAR binary search algorithm, implemented by the SAR controller 130,and generate the course stage 10 digital output bits. As noted above,the ϕ_(c-stage) pulses are derived from the phi2 signals. Thus, theϕ_(c-stage) pulses that function to generate the course stage 10 digitaloutput bits are generated while phi1 is low. In other words, the coursestage 10 is disabled (or powered down) after generating the MSBs of therespective first or second ADC stage 10, 20, thereby reducing powerconsumption.

FIG. 8 illustrates further details of an example of the fine stage 120.The course stage 110 includes a plurality of capacitors 160. Switches162 selectively connect one side of the capacitors 160 to the analoginput voltage signal V_(IP) or the V_(REF) signal (based on switches164), or ground. The other side of the capacitors 160 are selectivelyconnected to the common mode voltage signal V_(CM) based on switches166. The switches 164 and 166 operate in response to the phi1 signal,and the switches 166 operate in response to the ϕ_(F-stage) pulses shownin FIG. 6.

Thus, during phi1, the analog input voltage signal V_(ip) is on the topplates of the capacitors 160. At the same time, the bottom plates of thecapacitors 160 and the inputs of the fine comparator 122 are coupled tothe common mode voltage V_(cm).

During the next phase, the ϕ_(F-stage) pulses are asserted (after theϕ_(c-stage) pulses) to control the SAR binary search algorithm andgenerate the fine stage digital bits S2. The ϕ_(F-stage) is active onlyfor a portion of the time while phi1 is low. In other words, the finestage 120 is disabled (or powered down) when the course stage 110 isgenerating the MSBs of the respective first or second stage 10, 20,which helps to reduce power consumption.

FIG. 9 is a flow diagram illustrating an example ADC method 200 inaccordance with some embodiments. The illustrated method 200 includesreceiving an analog input voltage signal in block 210, such as the inputsignal V_(IP) received via the input terminal 102 as shown in FIG. 1. Inblock 212, the analog input voltage signal is converted to a firstnumber of bits S1 of a first digital value, and in block 214 the analoginput voltage signal is converted to a second number of bits S2 of thefirst digital value based at least in part on the first number of bitsof the first digital value. As discussed herein above, in someembodiments the S1 bits conversion is done by a course stage ADC and theS2 bits conversion is done by a fine stage ADS of a first stagesubranging ADC. The fine stage receives the S1 bits output by the coursestage, which may be the MSBs of the first subranging ADC stage. The S1and S2 bits are combined into a first digital value in block 216, whichrepresents a portion of a digital output signal corresponding to theanalog input voltage signal.

An analog residue signal based on a difference between the first digitalvalue and the analog input voltage signal is determined in block 218,and the analog residue signal is converted to a third number of bits S3(MSBs) of a second digital value in block 220. The analog residue signalis converted to a fourth number of bits S4 (LSBs, S4>S3) of the seconddigital value based at least in part on the third number of bits of thesecond digital value in block 222, and the third number of bits S3 andthe fourth number of bits S4 are combined into the second digital valuein block 224. In block 226, the first digital value and the seconddigital value are combined into a digital output signal representing theanalog input voltage signal.

Additionally, in some examples, a SAR ADC arrangement is employed, soconverting the analog input voltage signal to the first number of bitsof the first digital value in block 212 and converting the analog inputvoltage signal to the second number of bits of the first digital valuein block 214 each include comparing the analog input voltage signal to areference voltage range, and successively narrowing the referencevoltage range in response to the comparison. Moreover, in some examples,the analog input voltage signal is converted to the first number of bitsS1 of the first digital value by the coarse ADC stage 110, and theanalog input voltage signal is converted to the second number of bits S2of the first digital value by the fine ADC stage 120.

Disclosed embodiments include an ADC that includes an input terminalconfigured to receive an analog input voltage signal, and a first ADCstage coupled to the input terminal configured to output a first digitalvalue corresponding to the analog input voltage signal. The first ADCstage further outputs an analog residue signal corresponding to adifference between the first digital value and the analog input signal.A second ADC stage is coupled to the first ADC stage and is configuredto convert the analog residue signal to a second digital value. At leastone of the first ADC stage and the second ADC stage has a firstsub-stage configured to convert an analog signal to a first number ofbits of a digital value representing the analog signal, and a secondsub-stage configured to convert the analog signal to a second number ofbits of the digital value, where the second number of bits is greaterthan the first number of bits. A controller is coupled to the first andsecond ADC stages and is configured to combine the first digital valueand the second digital value into a digital output signal representingthe analog input voltage signal.

In accordance with further disclosed embodiments, an ADC includes aninput terminal configured to receive an analog input voltage signal andan output terminal configured to output a first digital output signalrepresenting the analog input voltage signal. A controller is configuredto receive a clock signal, and the controller is configured to establishfirst and second operation phases in response to the clock signal. Afirst ADC stage includes a coarse stage coupled to the input terminaland a fine stage coupled to the input terminal. The fine stage receivesan output of the coarse stage. The controller is configured to operatethe coarse stage to sample the analog input voltage signal during thefirst operation phase, operate the fine stage to sample the analog inputvoltage signal during the first operation phase, operate the coarsestage to convert the analog input voltage signal to a first number ofbits of the first digital output signal during the second operationphase, operate the fine stage to convert the analog signal to a secondnumber of bits of the first digital output signal during the secondoperation phase, and combine the first number of bits and the secondnumber of bits.

In accordance with still further disclosed embodiments, an ADC methodincludes receiving an analog input voltage signal. The analog inputvoltage signal is converted to a first number of bits of a first digitalvalue, and the analog input voltage signal is converted to a secondnumber of bits of the first digital value based at least in part on thefirst number of bits of the first digital value. The first number ofbits and the second number of bits are combined into the first digitalvalue. An analog residue signal is determined based on a differencebetween the first digital value and the analog input voltage signal. Theanalog residue signal is converted to a third number of bits of a seconddigital value, and the analog residue signal is converted to a fourthnumber of bits of the second digital value based at least in part on thethird number of bits of the second digital value. The third number ofbits and the fourth number of bits are combined into the second digitalvalue, and the first digital value and the second digital value are intoa digital output signal representing the analog input voltage signal.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An analog-to-digital converter (“ADC”),comprising: an input terminal configured to receive an analog inputvoltage signal; a first ADC stage coupled to the input terminal andconfigured to output a first digital value corresponding to the analoginput voltage signal and an analog residue signal corresponding to adifference between the first digital value and the analog input signal;a second ADC stage coupled to the first ADC stage and configured toconvert the analog residue signal to a second digital value, wherein thesecond ADC stage includes a first sub-stage configured to convert theanalog residue signal to a first number of bits of the second digitalvalue representing the analog reside signal, and a second sub-stageconfigured to convert the analog residue signal to a second number ofbits of the second digital value, where the second number of bits isgreater than the first number of bits; and a controller coupled to thefirst and second ADC stages and configured to combine the first digitalvalue and the second digital value into a digital output signalrepresenting the analog input voltage signal.
 2. The ADC of claim 1,wherein the first ADC stage includes a first sub-stage configured toconvert the analog input voltage signal to a first number of bits of thefirst digital value, and a second sub-stage configured to convert theanalog input voltage signal to a second number of bits of the firstdigital value, and wherein the second sub-stage is configured to outputthe analog residue signal.
 3. The ADC of claim 1, further comprising anamplifier receiving the analog residue signal output by the first ADCstage, wherein the amplifier is configured to apply a predetermined gainto the analog residue signal and output the amplified analog residuesignal to the second ADC stage.
 4. The ADC of claim 1, wherein thecontroller is configured to apply power to the first sub-stage during afirst operation phase where the first sub-stage samples the analog inputvoltage signal, and wherein the controller is configured to remove powerfrom the first sub-stage following the first operation phase.
 5. The ADCof claim 4, wherein the controller is configured to apply power to thesecond sub-stage during a second operation phase where the firstsub-phase generates the first number of bits and wherein the secondsub-phase generates the second number of bits.
 6. The ADC of claim 5,further comprising a clock signal generator configured to output a clocksignal, wherein the controller establishes the first and secondoperation phases based on the clock signal.
 7. The ADC of claim 4,further comprising an amplifier receiving the analog residue signaloutput by the first ADC stage, wherein the amplifier is configured toapply a predetermined gain to the analog residue signal and output theamplified analog residue signal to the second ADC stage, wherein thecontroller is configured disable the amplifier during the firstoperation phase.
 8. The ADC of claim 1, wherein the second sub-stage isconfigured to receive the first number of bits of the first digitalvalue.
 9. The ADC of claim 1, wherein the first ADC stage includes apipelined subranging SAR ADC.
 10. An analog-to-digital conversion(“ADC”) method, comprising: receiving an analog input voltage signal;converting the analog input voltage signal to a first number of bits ofa first digital value; converting the analog input voltage signal to asecond number of bits of the first digital value based at least in parton the first number of bits of the first digital value; combining thefirst number of bits and the second number of bits into the firstdigital value; determining an analog residue signal based on adifference between the first digital value and the analog input voltagesignal; converting the analog residue signal to a third number of bitsof a second digital value; converting the analog residue signal to afourth number of bits of the second digital value based at least in parton the third number of bits of the second digital value, wherein theanalog residue signal is converted to the third number of bits of thesecond digital value by a coarse ADC stage, and wherein the analogresidue signal is converted to the fourth number of bits of the seconddigital value by a fine ADC stage, and wherein the fourth number if bitsis greater than the third number of bits; combining the third number ofbits and the fourth number of bits into the second digital value; andcombining the first digital value and the second digital value into adigital output signal representing the analog input voltage signal. 11.The method of claim 10, wherein converting the analog input voltagesignal to the first number of bits of the first digital value, andconverting the analog input voltage signal to the second number of bitsof the first digital value each include comparing the analog inputvoltage signal to a reference voltage range, and successively narrowingthe reference voltage range in response to the comparison.
 12. Themethod of claim 10, wherein the analog input voltage signal is convertedto the first number of bits of the first digital value by a coarse ADCstage, and wherein the analog input voltage signal is converted to thesecond number of bits of the first digital value by a fine ADC stage,and wherein the second number if bits is greater than the first numberof bits.
 13. The method of claim 12, wherein receiving the analog inputvoltage signal includes sampling the analog input voltage signal duringa first operation stage, wherein the analog input voltage signal isconverted to the first number of bits of the first digital value duringa second operation phase, and wherein the coarse ADC stage is powereddown during the second ADC stage.
 14. An analog-to-digital converter(“ADC”), comprising: an input terminal configured to receive an analoginput voltage signal; an output terminal configured to output a firstdigital output signal representing the analog input voltage signal; acontroller configured to receive a clock signal, wherein the controlleris configured to establish first and second operation phases in responseto the clock signal; a first ADC stage coupled to the input terminal andconfigured to output a first digital value corresponding to the analoginput voltage signal and an analog residue signal corresponding to adifference between the first digital value and the analog input signal;a second ADC stage coupled to receive the analog residue signal, thesecond ADC stage including a coarse stage and a fine stage coupled toreceive an output of the coarse stage, the second ADC stage configuredto output a second digital value corresponding to the analog residuesignal; wherein the controller is configured to: operate the coarsestage to sample the analog residue signal during the first operationphase; operate the fine stage to sample the analog residue signal duringthe first operation phase; operate the coarse stage to convert theanalog residue signal to a first number of bits of the second digitaloutput signal during the second operation phase; operate the fine stageto convert the analog residue to a second number of bits of the seconddigital output signal during the second operation phase; and combine thefirst number of bits and the second number of bits.
 15. The ADC of claim14, wherein the second number of bits is greater than the first numberof bits.
 16. The ADC of claim 14, further comprising an amplifiercoupled to receive the analog residue signal.
 17. The ADC of claim 16,wherein the amplifier is configured to apply a predetermined gain to theanalog residue signal and output the amplified analog residue signal tothe second ADC stage.
 18. The ADC of claim 14, wherein the first ADCstage includes a coarse stage coupled to the input terminal, and a finestage coupled to receive an output of the coarse stage of the first ADCstage.
 19. The ADC of claim 14, wherein the first ADC stage includes apipelined subranging SAR ADC.
 20. The ADC of claim 14, wherein thecontroller is configured to apply power to the first sub-stage duringthe first operation phase, and wherein the controller is configured toremove power from the first sub-stage following the first operationphase.